1. Field of the Disclosure
The present invention relates to semiconductor devices, and, more particularly, to a CMOS image sensor and a method for fabricating the same.
2. Discussion of the Related Art
In general, image sensors are semiconductor devices for converting an optical image to electrical signals. Image sensors may be classified as a charge coupled device (CCD) and the CMOS (Complementary Metal Oxide Silicon) image sensor.
The CMOS image sensors widely used currently are switching type devices in which MOS transistors are fabricated on a semiconductor substrate as unit pixels using a CMOS technology, in which control circuits and signal processing circuits are peripheral circuits. The control circuits and signal processing circuits detect outputs of the unit pixels in succession using the MOS transistors.
That is, the CMOS image sensor has a photodiode and a plurality of MOS transistors in the unit pixel for detecting and/or transmitting the electric signal of each unit pixel in succession by a switching system to produce an image.
Since the CMOS image sensor uses a CMOS fabrication technology, the CMOS image sensor is advantageous in low power consumption, a simple fabrication process owing to a small number of photolithographic process steps, and so on.
A related art CMOS image sensor will be described with reference to the attached drawings, in which FIG. 1 illustrates a partial cross-section of a related art CMOS image sensor.
An insulating film 101 (e.g., a gate insulating film or an interlayer insulating film) is formed on a semiconductor substrate 100, and a first protective film 102 (e.g., an oxide film or a nitride film) is formed on an entire surface of the insulating film 101.
A thermal resin or thin TEOS layer is deposited on an entire surface of the semiconductor substrate 100 for protecting a micro-lens formed in a successive step, to form a second protective film 103, and a silicon nitride film or a silicon oxynitride film is deposited on an entire surface of the second protective film 103, to form a planarizing layer 104.
Then, a color filter layer 105 is formed on the planarizing layer 104 over every photodiode region (not shown). Each of the color filters in the color filter layer 105 is formed by coating a color resist and photolithographically etching the color resist with a mask provided separately.
Then, an overcoating layer 106 is formed on an entire surface of the substrate having the color filter layers 105.
A polymer is adhered onto the overcoating layer 106 as a microlens material layer. Then, the polymer (e.g., a photo-sensitive film) is patterned by exposure and development, to define microlens regions. The microlens material layer of the polymer is patterned selectively, to form a microlens pattern over the color filter layer 105. The microlens pattern is subjected to reflow heat treatment to form hemispherical microlenses 107 having a fixed curvature.
However, in this microlens forming method, the planarizing layer on the color filter layer reduces optical efficiency.
Moreover, the planarizing layer increases a distance between the photodiode and the microlens to be formed in a following step by the thickness of the planarizing layer, thereby failing to maximize the sensitivity of the CMOS image sensor.